This invention relates to a sputtering target, and in particular relates to an improvement in a composite sputtering target divided into target elements of a plurality of types arranged on a substrate.
Conventionally, the sputtering technique has been widely used as a technique for forming thin films on specific substrates. Apart from thin films consisting of a single constituent, various attempts have been made to form thin films of composite constituents, such as alloys. In such sputtering techniques, use is made of a composite sputtering target, which is divided into a plurality of respective constituents (see for example, early Japanese patent publication No. SHO.59-179783, of the present applicants). Such composite sputtering targets are used for example when it is difficult to produce a target having a composition ratio that departs beforehand from the stoichiometric amounts and yet which has a uniform composition, as for example with molybdenum and silicon, or in the case where there is a large difference between the melting points or vapor pressures, making it difficult to produce an alloy target body of the prescribed composition ratio.
Known forms of such composite target include disc-shaped targets formed by assembling a plurality of wedge-shaped target elements arranged alternately, or targets in the form of a rectangular plate, consisting of an alternate arrangement of strip-shaped target elements.
Examples of commonly used sputtering targets that are composite targets are shown in FIGS. 3 to 7. As shown in FIG. 3, a composite target 11 consists of two types of target elements 12 and 13 assembled alternately in mosaic form to produce an integral disc-shaped composite target. In this case, target elements 12 and 13 are respectively fan-shaped as shown in FIG. 4. The composite target 11 is mechanically fixed (FIG. 6) onto a copper backing plate 16 constituting the substrate by an inner junk ring 14 and outer junk ring 15 shown in FIG. 5. The usual fixing method is clamping by means of screws. In more detail, an axial cross-section of FIG. 6 is shown in FIG. 7.
Film deposition using a composite sputtering target as described above has the advantage that the composition ratio of the film which is formed can be altered at will be altering the types of target elements and/or their combination.
However, with continuing use of a conventional composite target as described above, occasionally gaps are produced between the respective target elements 12 and 13. These gaps are formed by differences in the thermal expansion that results from heating of the target during use, and the repeated thermal distortion which this causes. In particular, such gaps are mostly produced towards the end of the life of the target. This results in the problem that, when sputtering is performed, the copper backing plate, which is the substrate, is sputtered from the gaps which are thus formed, at the same time as the sputtering of the target element metal. When this occurs, it is difficult to form a thin film of stable composition ratio, and there is a bad effect on the quality of VLSI products.